This invention relates to flextensional electroacoustic transducers and in particular to flextensional transducers which have been modified to reduce the resonance frequency of the transducer for a given transducer size, or alternatively to result in a smaller transducer for a given resonance frequency.
A basic form of a flexural-extensional (flextensional) acoustic transducer of the prior art is shown in end-view in FIG. 1. The transducer 10 comprises a transduction driver 11, which may be a magnetostrictive rod or a stack of electrostrictive ceramic elements in a housing 12 comprising end portions 13 and flexible shell portions 14. The driver 11 is maintained in a state of compression by the shell 12. In operation, a change in polarity of electrical energization of the driver 11 causes the ends 13 to alternately extend, as shown by the arrows 15, and to retract. Upon elongation of the ends 13, the flexing shell portion 14 moves inwardly as shown by direction arrows 16 by an amount substantially greater than the amount by which the end 13 has been displaced at the same instant and vice versa. The transducer 10 is normally used to propagate acoustic energy in a water environment, and it is found that the flextensional transducer 10 provides a good means for transferring energy from the high acoustic impedance of driver 11 to the surrounding water by converting the small motion of the end 13 of transducer 10 to a much larger motion of the broader surface of the flexing shell 14 of the transducer 10. Transducers of the type described above may be found in W. J. Toulis, U.S. Pat. Nos. 3,277,433 and 3,277,537 and of H. C. Merchant, U.S. Pat. No. 3,258,738.
In many applications, it is desired that the acoustic energy be produced at a lower frequency than the resonance frequency of the transducer 10. However, optimum transfer of energy to the surrounding water is obtained at the frequency of resonance. The frequency of resonance of the transducer 10 is higher than the natural resonant frequency of the shell 12 alone because the shell is constrained at the ends 13 and due to the stiffening effect of the driver 11. Attempts to reduce the natural resonance frequency of the transducer 10 by making the shell 14 longer between ends 13 and/or reducing the shell 14 wall-thickness usually produce an unsatisfactory transducer in terms of handling (weight and size). Electroacoustic efficiency and effective transducer coupling may also suffer from these attempts to lower the transducer resonance frequency.
The natural resonance frequency of the shell 14 of transducer 10 may be approximately determined by considering the shell 14 to be a plate which is clamped at two ends. Such a plate has a resonance frequency that is proportional to 8(3EI/L.sup.3).sup.1/2 where E is the modulus of elasticity of the material of the plate, I is the moment of inertia of the shell wall cross section, and L is the length of the plate between the rigid clamps. In the transducer 10, the length L is approximately the distance between regions 17 and 18 which are approximately the transition points in shell 14 for the change in the direction of motion 15 of the end 13 and the motion 16 of the shell 14.
Prior art transducers having more than two shells have the same limitation with respect to resonance frequency because the shell ends must bend at their point of attachment to the drive mechanism. Illustrative embodiments of such transducers are shown in J. Butler, U.S. Pat. No. 4,742,499, which are incorporated herein by reference.
Another prior art transducer design in which this invention may be incorporated is similar to that shown in isometric view as the cuspidate-shaped transducer 50 of FIG. 6. In its prior art form as disclosed in H. C. Hayes, U.S. Pat. No. 2,064,911, and more recently in Butler U.S. Pat. No. 4,742,499, the transducer comprises three or more radiating curved plates 23" which are rigidly attached to constraining blocks 51.
The curved plates or shells 23" of the prior art are rigidly attached to compression blocks 51 by bolting or welding with a substantially tangential relationship of the curved plates 23" and the surface 55 of the constraining blocks 51 to which they were attached. The plates 23" could also be formed integrally with the constraining blocks 51 as by machining from a solid block of stainless steel, for example. The drives 11 are in contact with a center block 52, and at their outermost portions in contact with constraining blocks 51. A screw thread 53 extending through the drives 11 in conjunction with their accompanying nuts 54 is tensioned by nuts 54 against the constraining blocks 51 thereby providing compression of the drives 11 for reasons well known to those skilled in the art. These screw threaded rods (known as a tie rod in transducer art) are often omitted in flextensional designs of the prior art where the external shell alone is sufficiently strong to provide compression on the driver. In the cuspidate-variety of designs (such as those of Hayes and Butler) the shells usually need assistance from tie rods to provide compression. Electrical activation of the drives 11 causes the curved plates 23" to become more-and-less planar depending upon the outward and inward motion, respectively, of the compression elements 51, respectively.